Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers. Generally speaking, a power amplifier amplifies an audio signal by taking energy from a power supply and controlling an audio output signal to match an input signal shape but with a larger amplitude.
One example of an audio amplifier is a class-D amplifier. A class-D amplifier (also known as a “switching amplifier”) may comprise an electronic amplifier in which the amplifying devices (e.g., transistors, typically metal-oxide-semiconductor field effect transistors) operate as electronic switches. In a class-D amplifier, a signal to be amplified may be converted to a series of pulses by pulse-width modulation (PWM), pulse-density modulation (PDM), or another method of modulation, such that the signal is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) is a function of the magnitude of the signal. After amplification with a class-D amplifier, the output pulse train may be converted to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in the class-D amplifier or a load driven by the class-D amplifier. Class-D amplifiers are often used due to the fact that they may be more power efficient than linear analog amplifiers, in that class-D amplifiers may dissipate less power as heat in active devices as compared to linear analog amplifiers. Typically, a PWM amplifier is chosen in order to provide accurate load voltage with desirable Total Harmonic Distortion (THD) and Power Supply Rejection Ratio (PSRR).
Generally speaking, an audio amplifier is used to drive a heavy load through output pins that are exposed, which may result in a short circuit in the load (e.g., personal audio device is dropped in water or a malfunction of headphones plugged into the personal audio device). When there is a short circuit, damage may occur to the driver stage of the amplifier if it is without protection. Recently, a need has arisen to compare a short load threshold impedance that is much closer to a normal operation load impedance.
Conventional solutions require additional power, area, and circuit complexity just to detect a very low impedance short. A need for more accurate and less complex short detection in the audio amplifier is desired. Conventional solutions include serial detection, which involves adding in series an accurate resistor and measuring the voltage across the resistor. Conventional solutions also include parallel current detection and digital detection. An example parallel current detection is accomplished and provided by U.S. Pat. No. 5,973,569 which mirrors and scales output stage current and compares to a reference. An example parallel voltage detection is accomplished and provided by U.S. Pat. No. 7,701,287 which samples output voltage and compares it to a reference. An example digital detection is performed and provided by U.S. Pat. No. 8,749,303 which detects a short circuit when a class-D amplifier output pulse is skipped; however, it assumes the PWM pulse is never skipped during normal operation of the class-D amplifier. More specifically, U.S. Pat. No. 8,749,303 does not address a design in which the output PWM period varies as the input power signal changes.